1. Field of the Invention
The present invention pertains to a device for the protection of mask cathode tubes from the earth's magnetic field.
It is known that the earth's magnetic field affects the path of the electron beams emitted by a cathode tube gun and may cause what is called a "misregister" in a trichromatic tube, namely a discrepancy between the impact, on the screen, of the beams of the three fundamental colors and the corresponding phosphors. The amplitude of the spurious deviation depends on the amplitude of the earth's magnetic field as well as on the geometry of the tube (namely the distance between the gun and the mask, a parameter known as "p" and the distance between the mask and the envelope, a parameter known as "q").
The earth's magnetic field acts through its three components, namely the vertical component .DELTA.Y, the horizontal lateral component .DELTA.X, and the horizontal axial component .DELTA.Z. The vertical component causes a leftward spurious deviation in the Northern hemisphere while the lateral component causes a spurious deviation of the point of impact, downwards or upwards depending on whether the cathode tube is facing east or west, and the axial component causes a rotational type of deviation.
2. Description of the Prior Art
In present-day cathode tubes, to prevent the effect of the earth's magnetic field, the tubes are shielded along most of the path of the beams. The shielding is obtained by the frame-mask unit and by a drawn metal screen set in the tube and fixed on the frame, taking a substantially cone-like shape.
The shielding acts in the following way: the external magnetic field realigns the limits of the Weiss domain in the material forming the screen, creating an induced magnetic field that tends to oppose the action of the external field which has given rise to it. If the material of the screen has high magnetic permeability like, for example, mumetal, the induced field totally opposes the external field. Hence, there is no longer any disturbing field inside the shielding. If a material with low magnetic permeability is used (for example, soft steel), chosen essentially for its low cost, the compensation is no longer total. For more efficient compensation, the material of the screen has to be de-magnetized with magnetic field which is initially intense, and gradually decreases. This de-magnetization locates the magnetization curve on the so-called anhysteretic curve thus giving a more intense magnetic field opposed to the earth's field and, hence, giving better shielding. This de-magnetization is done conventionally, with a coil fixed to the rear of the tube on the cone.
The Applicant has observed that the frame-mask unit has a shielding effect with respect to the lateral and axial horizontal components and the vertical component: the effects these three components are respectively diminished by about at least 90%, 50% and 60%. The introducion of a magnetic screen made of low-permeabilty material gives, after de-magnetization, better compensation for the effects of the axial horizontal component and the vertical component (with a decrease in effect of about 87% and 97% respectively), but this result is obtained to the detriment of compensation for the lateral horizontal component, the effects of which are diminished only by about 78%. The variations in the effects of these components are simply determined by measuring the corresponding shifts in registers.